In the related art, it has been known a configuration to measure thickness of a disk owing to wavelength of reflection light as irradiating the disk with light (see Patent document 1). It has been also known a configuration to measure hardness owing to variation of a magnetic characteristic value of stainless steel as flowing current to the stainless steel (see Patent document 2). It has been also known a configuration to measure hardness of an object-to-be-measured (i.e., a measurement target) by utilizing frequency variation of a vibrator (see Patent document 3).
Patent document 1: JP 2008-39789A
Patent document 2: JP 2005-188948A
Patent document 3: JP 1-189583A